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    <meta name="description" content="虚函数是动态绑定的基础，它通常在类的定义中进行申明，可以为类中的一般函数，析构函数等。需要注意的是，虚函数必须是非静态的成员函数，且由于要实现动态绑定，虚函数一般不声明为内联函数，因为编译器对内联函数的处理是静态的。 一般虚函数虚函数声明在基类中，派生类可以继承该虚函数，也可以覆写该虚函数，实现本类的特定功能。虚函数的声明方式如下，需要注意的是，虚函数声明只能出现在类定义的函数原型声明中，而不能在">
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          虚函数
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        <p>虚函数是动态绑定的基础，它通常在类的定义中进行申明，可以为类中的一般函数，析构函数等。需要注意的是，虚函数必须是<strong>非静态</strong>的成员函数，且由于要实现动态绑定，虚函数一般<strong>不声明为内联函数</strong>，因为编译器对内联函数的处理是静态的。</p>
<h1 id="一般虚函数"><a href="#一般虚函数" class="headerlink" title="一般虚函数"></a>一般虚函数</h1><p>虚函数声明在基类中，派生类可以继承该虚函数，也可以覆写该虚函数，实现本类的特定功能。虚函数的声明方式如下，需要注意的是，<strong>虚函数声明只能出现在类定义的函数原型声明中，而不能在成员函数实现的时候</strong></p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">virtual 函数类型 函数名(形参表);</span><br></pre></td></tr></table></figure>

<p>基类通过继承虚函数，除了可实现自身独特功能外，在外部调用时，可以通过基类对象指针调用派生类的函数。</p>
<p>例如：基类Base中有虚函数display，Base类派生出A类和B类，A类和B类均重写了display函数。此时可通过Base类指针调用A类和B类的函数</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line">//定义功能函数</span><br><span class="line">void fun(Base *ptr)&#123;		//形参为指向基类对象的指针</span><br><span class="line">	ptr-&gt;display();</span><br><span class="line">&#125;</span><br><span class="line">int main()&#123;</span><br><span class="line">	Base base1;</span><br><span class="line">	A a;</span><br><span class="line">	B b;</span><br><span class="line">	//函数调用，实现动态绑定</span><br><span class="line">	fun(&amp;base1);		//调用Base类中的display函数</span><br><span class="line">	fun(&amp;a);			//调用A类中的display函数</span><br><span class="line">	fun(&amp;b);			//调用B类中的display函数</span><br><span class="line">	</span><br><span class="line">	return 0;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h1 id="虚析构函数"><a href="#虚析构函数" class="headerlink" title="虚析构函数"></a>虚析构函数</h1><p>虚析构函数的声明语法为</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">virtual ~类名();</span><br></pre></td></tr></table></figure>

<p>为什么要使用虚析构函数呢？设有基类Base，基类的派生类A，如果通过Base的指针来删除A的对象：</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="type">void</span> <span class="title">fun</span><span class="params">(Base *b)</span></span>&#123;</span><br><span class="line">	<span class="keyword">delete</span> b;</span><br><span class="line">&#125;</span><br><span class="line"><span class="function"><span class="type">int</span> <span class="title">main</span><span class="params">()</span></span>&#123;</span><br><span class="line">	Base *b = <span class="keyword">new</span> <span class="built_in">A</span>();</span><br><span class="line">	<span class="built_in">fun</span>(b);</span><br><span class="line">	<span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>会出现只调用Base类的析构函数，却不调用A的析构函数的情况。这将导致派生类对象中动态分配的内存空间没有得到释放，造成<strong>内存泄漏</strong>。避免上述错误的有效方法是将Base类中的析构函数声明为虚函数：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">class Base&#123;</span><br><span class="line">public:</span><br><span class="line">	virtual ~Base();</span><br><span class="line">&#125;;</span><br></pre></td></tr></table></figure>

<p>此时再执行上述代码，派生类对象中动态申请的内存空间将被正确地释放。</p>
<h1 id="纯虚函数"><a href="#纯虚函数" class="headerlink" title="纯虚函数"></a>纯虚函数</h1><p>纯虚函数的声明格式为：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">virtual 函数类型 函数名(参数表)=0;</span><br></pre></td></tr></table></figure>

<p>声明为纯虚函数后，基类中就可以不再给出函数的实现部分(即使给出也无法实例化)，纯虚函数的函数体由派生类给出。</p>
<p>带有虚函数的类是<strong>抽象类</strong>。抽象类不能实例化，即不能定义一个抽象类的对象，但是可以定义一个抽象类的指针和引用，通过指针和引用，就能指向并访问派生类的对象，其操作方法与一般虚函数相同。</p>

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